The major objectives of this investigation are threefold, and are aimed at testing the hypotheses that the activity of brainstem thermosensitive neurons controls the level of body temperature, and that changes in body temperature produced by intracerebral microinjected amines are due to an action on these neurons. The first objective is to observe the spontaneous spike activity, and to test the thermal sensitivity of individual brainstem neurons of unanesthetized, euthermic (non-hibernating) ground squirrels (Citellus lateralis) and of unanesthetized ground squirrels during hibernation and arousal from hibernation. The second objective is to test the responsiveness of individual brainstem neurons to iontophoretically applied biogenic amines in unanesthetized, euthermic ground squirrels and in unanesthetized ground squirrels in hibernation and during arousal from hibernation. The third objective is to study, in unanesthetized euthermic and hibernating ground squirrels, the thermoregulatory effects of biogenic amines microinjected through chronically implanted cannulas into the rostral and mesencephalic portions of the brainstem known to contain amine-sensitive, thermosensitive neurons. BIBLIOGRAPHIC REFERENCES: Beckman, A. L., E. Satinoff, and T. L. Stanton. Characterization of midbrain component of the trigger for arousal from hibernation. Am. J. Physiol. 230:368-375, 1976. Beckman, A. L., T. L. Stanton, and E. Satinoff. Inhibition of the CNS trigger process for arousal from hibernation. Am. J. Physiol. 230:1018-1025, 1976.